Summary IB Biology Unit 7: Nucleic Acids (45 Pointer)
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Biology HL
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International Baccalaureate
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IB Biology Course Book
Detailed objective-by-objective summary notes for Topic 7: Nucleic Acids for IB Biology SL/HL. Structured by Chapters (7.1 to 7.3) and categorised under each "Application", "Nature of Science" and "Understanding" statement given by IBO. Written by an IB HL Biology student who graduated with a 45/4...
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Unit 7: Nucleic Acids
7.1 DNA Structure and Replication (cont’d from 2.6, 2.7)
ß Skills- Analysis of results of the Hershey and Chase experiment providing
evidence that DNA is the genetic material
Scientists knew chromosomes played a role in heredity. However,
chromosomes contained both proteins and nucleic acid, and hence both were
contenders to being genetic material
Until the 1940s, the view that proteins was the hereditary material was held
since it had great variety due to 20 subunits vs nucleic acids’ 4 subunits, and
was found to have many specific functions
It was known that viruses are infectious particles that binds to host cells and
inject their genetic material.
Bacteriophages = phages = viruses that infect bacterial cells
Hershey and Chase used T2 bacteriophages in their experiment due to its
simple structure
Hershey-Chase experiment:
o Culture viruses containing proteins with 35S and culture separately
viruses which contain DNA with radioactive 32P
o Infect bacteria separately with the 2 types of viruses
o Use a blender to separate the non-genetic component of the virus from
the cell containing virus genetic material
o Centrifuge the culture solution to concentrate cells in the pellet while
non-genetic component of virus is in supernatant. Heavier bacteria
cells that contain genetic material sediment at the bottom, in
comparison to lighter phages.
o Radioactivity of supernatant and pellet are measured.
Results and interpretation:
o For 32P bacteria-infected cell, pellet has high radioactivity and
supernatant has minimal
o For 35S bacteria-infected cell, pellet has low radioactivity and
supernatant has high radioactivity
o This shows that DNA was in the infected cell, hence showing that DNA
is the genetic material
, Unit 7: Nucleic Acids
ß Applications- Rosalind Franklin’s and Maurice Wilkin’s investigation of DNA
structure by X-ray diffraction
ß NOS- Making careful observations: Rosalind Franklin’s X-ray diffraction
provided crucial evidence that DNA is a double helix
When a beam of X-rays is directed at a material, most of it passes through
while some is scattered by particles in the material (diffraction)
X-ray’s wavelength makes it particularly sensitive to diffraction by biological
molecules including DNA.
Franklin’s experiment:
o Though DNA cannot be crystallised, Franklin produced pure samples
of DNA with molecules aligned in narrow fibres. The array was orderly
enough for a diffraction pattern to be obtained, rather than random
scattering.
o Franklin developed a high resolution camera containing X-ray film to
obtain very clear images of diffraction patterns.
From the diffraction patterns obtained, Franklin made a series of deductions
on DNA structure:
o Cross in the centre indicated helical structure of DNA
o Angle of the cross shape showed the pitch (angle steepness) of helix
o Distance between the horizontal bars showed turns of the helix to be
3.4nm apart
o Distance between middle and top of pattern showed there was a
repeating structure, with a distance of 0.34nm between the repeats.
This turned out to be the vertical distance between adjacent base pairs
in the helix (10 base pairs per turn of helix)
These results were critical in the discovery of DNA structure. However,
Watson was shown the best diffraction pattern and Franklin’s calculations
without her knowledge, allowing Watson and Crick to build a model and
publish results before Franklin could.
∑ DNA structure suggested a mechanism for DNA replication
, Unit 7: Nucleic Acids
The complementary base pairing in Watson and Crick’s model led to the
hypothesis of semi-conservative replication. This is so since the model
suggests that the genome can be determined by just one DNA strand
∑ Nucleosomes help to supercoil the DNA
ß Skills- Utilisation of molecular visualisation software to analyse the
association between protein and DNA within a nucleosome
Nucleosomes:
o A nucleosome consists of a central core of eight histone proteins
(octamer), with DNA coiled around the proteins. An additional H1
histone serves to bind the DNA to the core.
o An octamer is made of 2 copies of 4 histone types. Positively charged
(basic) amino acids attract the negatively charged (acidic) DNA.
o Each protein has an N-terminal tail extending from the core. Chemical
modification of the tail is involved in gene expression.
o DNA wraps twice around each nucleosome
Supercoiling:
o The association of histones with DNA contributes to supercoiling.
o Short sections of “linker” DNA connects one nucleosome to another.
o A chain of nucleosomes form a tight helical 30 nm fibre that further
coils into a 300nm supercoil
o Supercoiled structure is relatively unstable due to torsional strain.
Energy required to maintain structure is minimised by minimising the
number of twists and writhes in the structure.
Uses of supercoiling:
o Space efficiency: allows great lengths of DNA to be packed into a small
space; the nucleosome is an adaptation in eukaryotes to pack large
eukaryotic genomes.
o Control gene expression: supercoiled regions of DNA cannot be
expressed. They have to be uncoiled first for expression.
o Allows specialisation/ differentiation: by permanently supercoiling DNA
segments (heterochromatin)
o Mitosis and Meiosis: allows chromosomes to be mobile
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